1. Field of the Invention
The present invention relates to a suspension arm for suspension of a wheel on an automobile vehicle body.
2. Description of the Related Art
A so-called A-shaped suspension arm conventionally, comprises a main arm, including mounting portions integrally formed at opposite ends of an arm body and adapted to be mounted to a wheel and to a vehicle body, respectively, and a subsidiary arm including an arm connecting portion secured to the main arm and a mounting portion mounted to the vehicle body. The arm connecting portion and the mounting portion are integrally formed at opposite ends of an arm body of the subsidiary arm. A damper is mounted to the main arm. Such suspension arm functions to transmit a load from a wheel W to the damper D and the vehicle body B, while being subjected to a first bending force FB applied from the damper D in a direction intersecting a plane .alpha. including points of application of the forces (i.e., mounting portions J.sub.1, J.sub.2, J.sub.3, and Jd) and a second bending force F.alpha. applied from the wheel W in a longitudinal direction along the plane .alpha., as shown in FIG. 1. A forging or pressing technique has been conventionally used in independently forming the main arm and the subsidiary arm.
In this type of the suspension arm, various different external forces, having different directions of application, are applied to the arm portions. However, a suspension arm has not been proposed as yet, which has a structure such that a required rigidity and strength is efficiently provided to various portions of the arm to oppose such various external forces, while at the same time, achieving a reduction in weight of the arm itself.
For example, when an arm body, comprised of a rib framework including a plurality of ribs extending in a directions connecting the points of application of forces, is formed by forging, a metal mold is used which is opened and closed in a direction intersecting a plane passing opposite the outer main ribs R, R, of the arm body 1' at right angles in FIG. 10 (i.e., in a lateral direction as viewed in FIG. 10). In order to facilitate the release of a forged product from the mold, a given draft .theta. is established in that side of the product which extends in the opening and closing direction.
Thereupon, when the main arm Am having the damper mounting portion Jd, as shown in FIG. 1, is intended to be designed within a given dimension in a vertical direction, because a large force F.beta. due to a vertical load from the damper D, is applied to the arm body, as described above, it is necessary to increase the sectional coefficient of the arm body in order to oppose such force. Therefore, for example, sectional centers o' of the main ribs R, R, from a bending-neutral plane n in FIG. 10, are disposed outwardly to the utmost (i.e., at locations vertically spaced from each other). However, the draft .theta. is established in the side of each of the ribs R, R, as described, above, and it is difficult to form the rib, itself, while reducing its thickness t'. For this reason, it is impossible to space the sectional center o' of the main rib R sufficiently outwardly apart from the neutral plane n. This is an obstacle to the insurance of a sufficient rigidity and strength required for the arm body 1' to oppose the bending force F.beta. with limitations of a predetermined weight (sectional area) and dimension H. Even with regard to the subsidiary arm As, to which the bending force F.alpha. is mainly applied along the plane .alpha., the draft .theta. provided for forging is an obstacle, for the same reason, to the insurance of a sufficient rigidity and strength required for the subsidiary arm As, itself, or a coupled subsidiary arm/main arm assembly As, Am, to oppose the bending force F.alpha. with the limitations of the predetermined weight (sectional area) and dimension.
In addition, it is difficult to form, by forging, a configuration having largely different wall thickness at various portions. Hence, the entire rib thickness is determined in accordance with the thickness of a rib portion requiring a largest strength. Thus, a part of the rib portions are formed thicker than necessary. Therefore, it is difficult to provide a reduction in weight of each arm body.
Further, if a thin plate material, having a relatively large thickness, is produced by pressing and is used in order to provide a large sectional coefficient, a large wrinkle suppressing margin must be specially left. In addition, the material is present in the vicinity of the neutral plane of the arm body and hence, the arm body is not of a reasonable section. Thus, a separate step, such as punching, is required for a reduction in weight, and the working efficiency is inferior.